Fluorine has the highest electronegativity in the periodic table. As such, incorporation of fluorine into molecules often results in a significant change in the physical and chemical properties of molecules. Some fluorine-containing compounds have high electrochemical stability and are useful in electrochemical energy storage devices such as batteries and electric double layer capacitors (EDLCs). For example, fluorinated salts such as lithium bis(trifluoromethylsulfonyl)imide (Li TFSI) have been used as components of electrolytes for batteries and EDLCs. These salts have advantageous properties, including high thermal stability, and wide electrochemical stability windows, but these salts can cause corrosion of cell components. U.S. Pat. No. 5,916,475 discloses lithium bis(fluorosulfonyl)imide salts as having advantages over other electrolyte salts such as Li TFSI and Li PF6, including better temperature stability, higher conductivity, and lower corrosion rates. Due at least in part to these advantages there has been extensive research activity in synthesis, structural and electrochemical aspects of various fluorine containing compounds including bis(fluorosulfonyl)imide (HFSI), its metal salts and ionic liquids comprising such compounds.
Bis(fluorosulfonyl)imides and ionic liquids comprising the same have been shown to be useful as electrolytes in lithium ion batteries and ultracapacitors. Bis(fluorosulfonyl)imide is a relatively strong acid and forms various stable metal salts. These compounds are useful as electrolytes and the lithium salt of bis(fluorosulfonyl)imide (i.e., LiFSI) is particularly useful in batteries and ultracapacitors.
Lithium ion batteries are particularly attractive as a secondary battery due to their high energy density and high power density. Batteries with electrolytes comprising ionic liquids of bis(fluorosulfonyl)imide and/or its metal salt have shown to be safer, more reliable, and possess higher energy density relative to many conventional lithium ion batteries.
Ambient temperature ionic liquids are useful and safe electrolytes due to their non-volatility, non-flammability, wide electrochemical stability window and high ionic conductivity. Among various ionic liquids, bis(fluorosulfonyl)imide-based ionic liquids typically show a significantly lower viscosity, lower melting point and higher ionic conductivity than other ionic liquids. Some studies have also shown that a mixture of two different alkali metal salts of FSI (resulting in a eutectic mixture) has novel electrochemical properties. The eutectic point for LiFSI-KFSI is 338 K and that for NaFSI-KFSI is 330 K. The electrochemical window of LiFSI-KFSI eutectic melt is 6.0 V at 348 K and that of NaFSI-KFSI eutectic melt is 5.0 V at 340 K. These new inorganic ionic liquids are promising electrolytes for various high-temperature electrochemical applications.
Despite the advantages of compounds containing the bis(fluorosulfonyl)imide ion, no commercial production exists more than 45 years after its first synthesis, and nearly twenty years after its identification as a promising material for electrochemical applications. This is due at least in part to the cost and difficulty of synthesizing high-purity salts of the FSI anion. While many processes for producing HFSI are known, each of the known methods for synthesizing HFSI has disadvantages or short comings. For example, one method for synthesizing HFSI uses urea (NH2CONH2) and fluorosulfonic acid (FSO3H). One of the major disadvantages for this process is the toxicity and corrosive nature of FSO3H. Moreover, it is difficult to control this reaction due to local overheating during the addition of fluorosulfonic acid to the reaction mixture. This difficulty in controlling the reaction results in an unpredictable yield of the desired product. See, for example, Chem. Ber., 1962, 95, 246-248 (61% yield) and L. Zatloukalova, Thesis, UJEP Brno, 1979 (14.5% yield).
Another method for synthesizing HFSI involves fluorinating bis(chlorosulfonyl)-imide (i.e., HCSI) with arsenic trifluoride (AsF3). In this reaction, HCSI is treated with AsF3. Arsenic trifluoride is toxic and because it has a high vapor pressure, it is particularly difficult to handle on an industrial scale. A typical reaction uses 1:8.6 ratio of HCSI to AsF3. This means a large excess of highly dangerous arsenic trifluoride is used.
HFSI can also be prepared by the fluorination of HCSI with antimony trifluoride (SbF3). The antimony trichloride byproduct of this reaction has both high solubility in HFSI and a very similar boiling point, making it very difficult to separate from the desired product. The product of this reaction is also typically contaminated with chloride, which renders the product unsuitable for electrochemical applications.
Yet another method for producing HFSI involves reacting HCSI with excess anhydrous HF at high temperature. See, for example, U.S. Pat. No. 7,919,629. The yield of this reaction is at most 60%, with the product contaminated with fluorosulfonic acid that is produced from the decomposition of HCSI. This by-product is difficult to remove on an industrial scale as the boiling point is close to that of HFSI.
Synthesis of lithium and sodium salts of HFSI has been reported. See, for example, Electrochemical Society Transactions, 2009, 24, pp. 91-98; and Polyhedron, 2006, 25, pp. 1292-1298. In particular, lithium salt of HFSI was synthesized by the metathesis reaction of potassium salt of HFSI (i.e., KFSI) with lithium perchlorate (LiClO4). This reaction also produces potassium perchlorate (KClO4), which is an explosive compound. HFSI has also been synthesized from KFSI using perchloric acid. This reaction also produces KClO4. These processes are not suitable for commercial scale synthesis due to explosive nature of perchloric acid, lithium perchlorate, and potassium perchlorate.
U.S. Pat. No. 7,253,317 describes the synthesis of HFSI from bis(chlorosulfonyl)imide or HCSI, using potassium fluoride (KF) and other monovalent fluorides. This process is relatively slow (22 hours), typically requires volatile organic solvents (nitromethane) and yields a product with too high of potassium content for use in batteries. In addition, the reaction can form dangerous nitrous vapors in the reactor.
U.S. Pat. No. 7,919,629 describes the difficulty of synthesizing HFSI, stating in part “It thus appears that the use of bis(fluorosulfonyl)imide is particularly complex to implement. Despite intensive efforts for nearly ten years in collaboration with renowned academic and industrial experts in fluorine chemistry, production on an industrial scale of bis(fluorosulfonyl)imides could not be implemented.”
Accordingly, there is a need for a relatively safer and/or less costly method for producing high purity bis(fluorosulfonyl)imide compounds and derivatives thereof.